The present invention is related generally to the field of orthodontics. Particularly, the present invention is related to a dental appliance for repositioning teeth and a method of determining the useful life of such an appliance for this purpose.
Orthodontic treatments involve repositioning misaligned teeth and improving bite configurations for improved cosmetic appearance and dental function. Repositioning is accomplished by applying gentle controlled forces to the teeth over an extended period of time. Due to the limited space within the oral cavity and extensive movements that some teeth must undergo, the teeth will often be moved throughout a series of intermediate patterns to properly arrange the teeth. For example, molars may be temporarily moved backwards (distalized) to create adequate space for movement of the incisors. Thus, a single patient may experience an average of 25-30 stages or alignment patterns before achieving the final desired configuration.
Recently, it has been found that such repositioning may be accomplished with the use of a series of removable elastic positioning appliances. Such appliances comprise a thin shell of elastic material that generally conforms to a patient""s teeth but is slightly out of alignment with an initial or immediately prior tooth configuration. Placement of the elastic positioner over the teeth applies controlled forces in specific locations to gradually move the teeth into the new configuration. Repetition of this process with successive appliances comprising new configurations eventually move the teeth through a series of intermediate configurations or alignment patterns to a final desired configuration. A full description of an exemplary elastic polymeric positioning appliance is described in U.S. Pat. No. 5,975,893, and in published PCT application WO 98/58596 which designates the United States and which is assigned to the assignee of the present application. Both these documents are incorporated by reference for all purposes.
A patient generally uses a repositioning appliance, corresponding to a given stage in a treatment plan, until the appliance is no longer applying sufficient repositioning forces to the patient""s teeth. When a patient first places an appliance over their teeth, the misalignment of the appliance with the teeth will apply forces on the teeth at the points of contact. The larger the misalignment, the stronger the repositioning force. As the teeth gradually move into the desired configuration, the misalignment decreases and the applied force lessens. When the teeth substantially reach the desired configuration, the force may approach zero. It is at this point that the useful life of such an appliance for applying repositioning force has ended. The patient may then progress to the next stage in treatment plan and begin wearing the next successive elastic repositioning appliance. The new appliance will apply repositioning forces to move the teeth to the next desired configuration, repeating the appliance wear cycle.
Determining when to progress a patient to the next stage in a treatment plan is largely based on the discretion of the orthodontic practitioner. Gross misalignment of the appliance to the teeth of the patient may be evident by visual observation. However, as the teeth become more closely aligned with the appliance, the difference becomes less and less obvious. If a patient is advanced to the next stage in treatment prematurely, excessive forces may be applied to the teeth. Excessive force, in turn, may cause patient discomfort and possible biological consequences to the teeth and supporting tissue, such as undermining resorption. In addition, repeated premature progression may lead to poor fitting of the appliance and the application of undesired forces to the teeth. A poor fitting appliance may contact the teeth in unforeseen areas applying unprescribed forces to the teeth. The result may be undesired repositioning of the teeth.
Likewise, delay in progression to the next stage may unduly lengthen the treatment schedule, which may be unfavorable for both patient and practitioner. Lengthening of the treatment schedule may increase the possibility of treatment coinciding with other dental procedures, such as fillings, crowns, root canals, etc, which may interrupt the treatment plan. In addition, such lengthening may increase the possibility of tooth decay and/or periodontal disease in patients with poor dental hygiene.
Thus, it would be desired to provide dental appliances, systems and methods which may be used to more accurately assess and determine the useful life of such repositioning appliances. Such devices and methods should be economical, efficient and easy to use. In particular, they should provide indicators to assist in estimating the level of repositioning forces applied to the teeth when the appliance is placed over the teeth. At least some of these objectives, as well as others, will be met by the designs and methods of the present invention described hereinafter.
The present invention provides dental appliances, systems and methods which allow orthodontic practitioners to more easily monitor patient treatment progress. When using elastic repositioning appliances in orthodontic treatment, a patient may wear such an appliance until their teeth have repositioned to the tooth configuration prescribed by the appliance. At that point, the patient should progress to the next appliance for repositioning to the next tooth configuration in the treatment plan. Indications that the appliance is no longer supplying significant repositioning forces may be beneficial in the determination of progressing the patient to the next appliance or next stage in treatment. Mechanical stress in the material of the appliance, when the appliance is in place over the patient""s teeth, may serve as an indicator of repositioning force applied against the teeth. Such stress may be visualized with the use of photoelastic material in or on the appliance.
Photoelasticity is the effect that causes a material to become xe2x80x9cbirefringentxe2x80x9d when placed under stress. Birefringence may be described as the division of light into two components which is found in materials which have two different indices of refraction in different directions (i.e., when light entering certain transparent materials splits into two beams which travel at different speeds). The result is a fringe or interference pattern in the area under stress which may be observed in the presence of polarized light. Such patterns may be easily visually observed as an indication that the material is under mechanical stress.
In a first aspect of the present invention, a dental appliance comprises at least one layer of photoelastic material. In the case of an elastic repositioning appliance, the appliance comprises a shell having cavities shaped to receive teeth. The cavities may be misaligned with the patient""s current tooth configuration such that placement of the appliance over the teeth will apply forces on the teeth at the points of contact. It may be appreciated that the appliance may also apply forces to the teeth through various attachment devices and similar accessories. In all cases, mechanical stress will occur in the material of the appliance in the areas where force is applied. The photoelastic material in the appliance is thus caused to become birefringent under such stress providing interference patterns which may allow identification of these areas by an observer, typically by simple visual observation.
In a second aspect of the present invention, a dental appliance comprising photoelastic material may be constructed in a variety of configurations. In one embodiment, the shell of an elastic repositioning appliance is created by thermal forming sheet of polymer material over a model or cast of the patient""s teeth. Photoelastic material is also provided in a sheet form, and the photoelastic material is preferably thermal formed with the sheet of polymer to create the appliance. If the polymer material is itself photoelastic, the shell may be formed without the addition of a separate sheet of photoelastic material. Thus, the appliance may consist essentially of only a thermal formed sheet of photoelastic material, namely the polymer shell. In a further alternative embodiment, the photoelastic material may be applied to the appliance as a coating on the surface of the shell.
According to the methods of the present invention, all or portions of a dental appliance may be evaluated for the presence of mechanical stress as an indication of the progress of orthodontic treatment of a patient. As described above, a dental appliance comprises at least one layer of photoelastic material in at least a portion of the appliance. After the appliance is positioned over the teeth of a patient, polarized light is directed toward the designated area. Observation of interference patterns in the shell of the appliance (at least in the portion comprising the photoelastic material) indicates the presence of mechanical stress due to positioning of the appliance over the patient""s teeth.
In a first aspect of the methods of the present invention, if mechanical stress is present, light will be refracted in a manner which forms an interference pattern. However, it may not be evident whether the stresses are due to the application of repositioning forces to the teeth or result from inherent stresses in the appliance. For instance, the material of the appliance may be under stress when the appliance is in a free state, i.e. when the appliance is removed from the patient""s teeth and is not subjected to applied force. This may occur due to processing of the appliance or general wear on the appliance over time. For example, thermal forming of a photoelastic sheet may introduce permanent stresses into the walls of the appliance shell. Likewise, general use of the appliance may introduce plastic deformations in portions of the appliance forming permanent stresses in these areas. Therefore, the evaluation of interference pattern(s) in an appliance when placed over the patient""s teeth may require further methods to determine if the stresses are due to the application of repositioning forces. Thus, the methods of the present invention further provide that polarized light may be directed toward the appliance while the appliance is in a free state. If it is observed that interference pattern(s) are not present in the free state, it may be concluded that the appearance of interference pattern(s) while the appliance is positioned over a patient""s teeth are due to the continued application of repositioning forces and that the dental appliance is still useful in applying repositioning forces to the patient""s teeth. If, on the other hand, it is observed that an interference pattern(s) are present in the free state, a comparison between the interference pattern(s) in the free state and the patterns(s) when the appliance is positioned over the patient""s teeth should be made. If a change in the interference pattern is evident, it may be concluded that such a change is due to the application of repositioning forces. Therefore, it may be determined that the dental appliance is still useful in applying such forces.
In another aspect of the methods of the present invention, if mechanical stress is not present or is minimally present, light may be refracted in a manner which forms minimal or no interference pattern or an interference pattern which remains unchanged from the free state. Such observation indicates the absence of stress due to positioning of the appliance over the patient""s teeth. Therefore, the dental appliance is likely no longer useful in applying repositioning forces.
In a fourth aspect of the methods of the present invention, the usefulness of an elastic repositioning appliance in applying repositioning forces to a patient""s teeth may be monitored throughout orthodontic treatment. In a first embodiment, portions or all of a dental appliance may be evaluated as described above. Such methods may be repeated at predetermined intervals over time until absence of mechanical stress in the designated areas due to positioning of the appliance over the patient""s teeth is observed. This will indicate that the patient""s teeth have repositioned into the tooth arrangement prescribed by the appliance. Thus, at this point, the use of the appliance may be discontinued and replaced with another appliance having a different geometry selected to move the teeth to the next intermediate or final arrangement
In a second embodiment, all or portions of a dental appliance may be evaluated as described above, and the intensity of the mechanical stress due to positioning of the appliance over the patient""s teeth may be determined. This may be accomplished simply by noticing the size or specific characteristics of the interference pattern(s), or it may involve more detailed methods of photostress analysis. Such methods may be repeated at predetermined intervals over time until the intensity of the mechanical stress decreases to or below a threshold. This may indicate that the patient""s teeth have sufficiently repositioned into the tooth arrangement prescribed by the appliance. Thus, at this point, the use of the appliance may be discontinued for repositioning purposes.
Other objects and advantages of the present invention will become apparent from the detailed description to follow, together with the accompanying drawings.